The digital calipers fabricated by the principle of capacitive position sensing, with the advantages of high resolution, easy reading etc., have been widely applied, while the main drawback is that it should be used under purer environmental circumstances. Because there is an air gap between the electrodes of a capacitive position sensor and capacitance varies, when pollutants like oil, water, and dust etc. with dielectric constants differing from air are intruded into the air gap, resulted in the reliability and accuracy of the value measured will be affected. But in the working place of the machinery industry, the existence of the pollutants like cutting oil, cooling water and dust etc. are inevitable, which resulted in unreliable working of capacitive digital calipers. Accordingly, their spreading and application are largely restricted.
In order to overcome the drawbacks of capacitive position sensor which is sensitive to the pollutants, various solutions have been proposed in the prior art. For example, in U.S. Pat. No. 5,172,485 Gerhard et al proposed a non-air-gap method by setting the sliding contact of the upper, lower support members of electrodes of the capacitive position sensor. Theoretically speaking, there were no pollutants can be intruded, since no air gap existed between the two electrodes. However, unless sliding contact was frictionless, insulation protecting layers of the electrodes shall be worn by sliding contact. The electrode contact surfaces must be extremely flat and smooth to meet the requirements of no air gap and also no friction, which were rather difficult to be reached. Even if the requirements could be achieved in fabrication, air gaps can also be formed partly between the electrodes due to expansion and contraction by heat and cold, stress deformation and abrasion etc. As long as a local air gap exists, abrasion shall be aggravated by dust intrusion and the air gap will be increasingly expanded. Consequently, as a matter of fact, the problem in sensitivity of the capacitive position sensor to pollutants still cannot be solved by the sliding contact method. In China Patent CN1114412A, Andermo proposed a method of covering the electrodes with a thicker dielectric layer for reducing sensitivity of capacitive position sensor to the pollutants, it decreased the capacitive reactance change rate of the sensor caused by pollutant intruded into the air gap and weakened over one half of sensitivity of the capacitive position sensor to the pollutants. However, the above-mentioned problem was still not yet perfectly solved. The reliability and accuracy of the value measured can still be affected by the pollutants intruded. Another method was disclosed by Andermo et al in China Patent CN1147084A, it comprised an electrostatic capacitive slide caliper adopting an entirely sealed capacitive position sensor to prevent pollutants from intruding. Since the electrode length of linear position sensor on the main beam must not be shorter than the range of the caliper and the shorter electrode on the slider should move correspondingly to the longer electrode, the sealing of capacitive position sensor was a quite difficult problem of long-range motive seal. The method of Andermo et al was to place both the longer and the shorter electrodes in an enclosed cavity, compressed the shorter mobile electrode on the longer fixed electrode fastened to the main beam by a clip spring and bring the mobile electrode plate to slide on the fixed electrode plate by a rod connected to the slider via a through-hole on the sealed cavity. As for the method, rigidity of the connecting rod of the slider and mobile electrode plate was only required along the axial direction of the slider movement, while free bending in other directions is allowed, so as to prevent the capacitive position sensor from damaging due to over pressure loaded on the electrode plate. This is a nearly unreachable requirement since no material at present is of unidirectional rigidity and also free-bending property along other directions. The axial length will certainly change in a certain extent when the bending deformation of the material takes place. In the three embodiments of this patent, an elastic component 172, a bending component 672 or a clip spring 772 were respectively connected to the position between the mobile electrode plate and the rigid rod fastened on the slider. The application of these resilient components was solely intended for realizing the cushion purpose of flexure to avoid overpressure on other parts, whereas axial rigidity of the connecting rod was not really guaranteed because of the fact that as long as the rod displacement perpendicular to axial direction surpasses 0.1 mm, its length variation along the axial direction of slider will exceed 0.01 mm. This length variation changed randomly according to user mode, which causes mismatching of one-to-one correspondence between the positions of slider and the mobile electrode of the sensor. So measurement errors shall be resulted in and the caliper accuracy cannot be guaranteed. The spreading and application of the method disclosed in the said patent was also restricted due to the such problems as wear-prone property of electrodes, non-removable electromagnetic interference due to over long detection electrode, and difficulties in production due to complicated structure etc.